1. Field of the Invention
The present invention is directed to an anaesthetic machine of the type having a gas flow system with a variable-volume gas container for breathing gas, a fresh gas system for supplying a flow of fresh gas to the gas flow system, and a control unit which controls the variable-volume gas container during a first phase of inspiration so that a flow of breathing gas is released and mixes with a flow of fresh gas from the fresh gas system.
2. Description of the Prior Art
Anaesthetic machines usually operate in a manner referred to as volume-controlled modes, i.e. they are devised to supply a specific tidal volume of breathing gas to a patient in every breathing cycle. This can be achieved by utilization of a variable-volume gas container whose volume corresponds to the tidal volume. The gas container can be a piston or a bellows that is mechanically restricted to the specific tidal volume. One such system is described in German OS 29 45 472.
Anaesthetic machines that are also capable of operating in pressure-controlled modes have been described, e.g. in U.S. Pat. No. 5,315,989. Here, the variable-volume gas container is an unrestricted bellows. The pressure of drive gas applied to the exterior of the bellows is regulated in order to maintain a specific inspiratory or expiratory pressure.
A fresh gas is usually also introduced downstream from the variable-volume gas container. This flow of fresh gas is usually constant and continuous. The flow of fresh gas naturally has an impact on the flow of gas from the variable -volume gas container. The volume of this fresh gas must be compensated in the volume-control mode.
Since gases are compressible, minimizing the volume of gas in the anaesthetic machine is desirable. Moreover, anaesthetic agents are generally expensive, so minimizing the consumption of gas in the anaesthetic machine is also desirable.
Known commercial systems are unable to achieve these advantages.
An object of the present invention is accordingly to achieve an anaesthetic machine which is capable of operating in all known ventilation modes, without any real limitations, for both anaesthesia and intensive care, especially in the pressure-controlled mode.
Another object of the invention is to achieve an anaesthetic machine that solves the aforementioned gas consumption problems.
The above object is achieved in accordance with the principles of the present invention in an anaesthetic machine of the type initially described wherein the control unit starts a second phase of inspiration if the flow of breathing gas from the variable-volume gas container ceases before inspiration is completed, and in this second phase the control unit controls the anaesthetic machine so that inspiration is maintained by a flow of fresh gas from the fresh gas system.
A number of advantages is achieved by the invention. Among other things, a smaller volume can be kept in the gas container, with no risk of the patient running out of gas or of any failure of the breathing mode to operate as intended. If, during the first phase of inspiration, gas flow from the variable-volume gas container ceases, a control unit operates to switch operation to a second phase of inspiration. During the second phase inspiration is maintained by gas from the fresh gas system only.
In principle, flow from the variable-volume gas container could cease for two different reasons. One is because no more gas remains in the variable-volume container (minimized system) or because the flow of fresh gas is large enough to sustain inspiration, whereupon flow from the variable-volume gas container has been stopped by the control unit.
The former would be more common in systems wherein a control unit in the anaesthetic machine controls flows from both the variable-volume container and the fresh gas source.
The latter would be more common in systems wherein the setting of a continuous flow of fresh gas is manually controlled by a physician. The control unit would then control flow of gas in other parts of the anaesthetic system for maintaining set inspiration characteristics.
Any surplus breathing gas can be used to fill the gas container. The patient therefore receives a larger percentage of breathing gas in the next breathing cycle.
If the gas container becomes completely filled during the second phase before inspiration has concluded, the surplus can be evacuated during a third phase.
If the flow of gas into the patient increases during the second or third phase of inspiration so the flow of fresh gas is not enough to sustain inspiration, the anaesthetic machine can revert to the first phase and supply gas from the gas container.
The anaesthetic machine can be regulated in numerous ways in order to achieve the effects of the second and third phases.
One way is to devise a control unit to exercise active control of the flow of fresh gas so this flow is big enough (to sustain inspiration and possibly even fill the gas container to a specific volume).
Another way is to arrange one or more valves at strategic locations in the anaesthetic machine, preferably at the gas container, and to regulate them so as to maintain inspiration.
The flow of fresh gas could then be allowed to remain constant and (possibly) continuous.
The anaesthetic machine according to the invention is especially suitable for use in pressure-controlled modes, whereby a pressure gauge is applied to or near the patient in order to measure patient pressure.